1. Field of the Invention
The present invention relates to a snubber energy regenerating circuit for regenerating an energy of a snubber circuit in order to prevent an overvoltage or too large dv/dt of a voltage applied to a semiconductor element in a power converter.
2. Description of the Related Art
A power converter using semiconductor elements is provided with a snubber circuit so as to prevent the semiconductor elements from being broken due to an overvoltage or a voltage which rises fast (large dv/dt). Capacitance Cs of a capacitor in the snubber circuit is required to be increased since recent semiconductor elements have a high widthstand voltage and a large current. For example, in a 4,500 V, 2,500 A gate turn-off thyristor (to be referred to as a GTO hereinafter), a capacitor of Cs=6 .mu.F is used. When the thyristor is used at power supply voltage V=2,000 V, the power supply voltage is charged/discharged on/from a snubber capacitor upon switching of the GTO. In this case, when switching frequency f of the GTO is 50 Hz, power Pcs upon charging/discharging is as high as: ##EQU1## If this power is consumed by, e.g., a snubber resistor as a loss, this causes a decrease in efficiency of the apparatus. Then, the outer shape of the snubber resistor becomes considerably large, and a large amount of heat is generated by the resistor. Therefore, the apparatus cannot be made compact.
FIG. 2 shows a conventional snubber energy regenerating circuit. Since upper and lower arms are equivalent to each other, the upper arm will be described below. A series circuit of snubber capacitor 31 and snubber diode 21 is connected in parallel with switching element 11, and voltage transformer 51 is connected in parallel with diode 21. The secondary winding of voltage transformer 51 is connected to a DC power supply bus through diode 41. The operation of this circuit will be explained with reference to FIG. 3.
Assume that switching element 11 is kept off. In this case, a power supply voltage is charged on snubber capacitor 31. When switching element 11 is turned on, a charge on snubber capacitor 31 is discharged through voltage transformer 51 and semiconductor element 11. The turn ratio of the windings of voltage transformer 51 is set so that a voltage higher than the DC power supply voltage is induced in the secondary winding of voltage transformer 51. Thus, diode 41 is enabled by discharging, and energy on snubber capacitor 31 is regenerated to a DC power supply.
FIG. 4 shows a circuit which could be considered as a modification of FIG. 2. In place of voltage transformers 51 and 52 in FIG. 2, current transformers 81 and 82 are used. The operation of this circuit is basically the same as that of FIG. 2.
The snubber energy regenerating circuit described above theoretically includes no element which consumes large energy, and is a good system. However, this circuit requires a relatively expensive transformer for each switching element. In addition, when a circuit such as a three-phase bridge inverter using a large number of switching elements is considered, the circuit arrangement is complicated.